Why pump your septic tank??
There is one basic reason for routine maintenance (i.e. pumping) of your septic system.
REMOVE INDIGESTABLE SOLIDS. As explained in "How a septic system works", the septic tank is a solids collection system. The more solids it collects, the less efficient it becomes, and the more solids are passed with the effluent to the drainage system. As the tank becomes overfull with solids, the tank may plug (thus backing up sewage into your home), or very little drainage system damaging solids are collected by the tank.
Also, while the tank is open, you septic system should be inspected for damage (concrete corrosion or baffles missing/corroded) or impending drainage system failure.